1. Field of the Invention
The present invention relates to an electrode employing a nitride-based semiconductor of III–V group compound that is formed as an ohmic electrode of low contact resistance, and a producing method thereof.
2. Description of the Background Art
A nitride-based semiconductor of III–V group compound is excellent in thermostability and allows control of a bandgap width by changing its composition. Thus, it can be used for various semiconductor devices including light-emitting devices and high-temperature devices.
Conventionally, in an electrode employing a nitride-based semiconductor of III–V group compound, a multilayered structure has widely been used to ensure a favorable ohmic characteristic of low contact resistance for the nitride-based semiconductor of III–V group compound. It is reported in Japanese Patent Laying-Open No. 5-291621 that an electrode employing a gallium nitride doped with an n type impurity or non-doped GaN as the nitride-based semiconductor of III–V group compound and combined with a multilayered structure of Ti and Al exhibits a favorable ohmic characteristic with low contact resistance of 1×10−7 Ωcm2 to 1×10−6 Ωcm2.
In the above-described case, however, if AlGaN is employed as the nitride-based semiconductor of III–V group compound and it is combined with a multilayered structure of Ti and Al, the contact resistance increases along with an increase of a content ratio of Al within the group III metals in the semiconductor (hereinafter, referred to as the “Al composition ratio”). This is because the bandgap of AlGaN itself becomes large when AlGaN of a high Al composition ratio is employed, which leads to an increased potential barrier with respect to a metal. It is also reported, in Appl. Phys. Lett. Vol. 73 (1998), pp. 2582–2584, that the contact resistance becomes 2.1 ×10−3 Ωcm2 when the Al composition ratio is 22%, with which the favorable ohmic characteristic cannot be expected.